1. Field of the Invention
The present invention relates generally to a signal level adjustment method, a signal level adjustment system and a signal level adjustment circuit, and more particularly to a signal level adjustment method, signal level adjustment system and signal level adjustment circuit capable of adjusting pre-emphasis intensity to a proper value in accordance with transmission line characteristics.
2. Description of the Related Art
In recent years, in semiconductor integrated circuit devices such as LSI, the transmission of data transmitted/received between the semiconductor integrated circuit devices has been speeded up in accordance with the high operating frequency. However, the semiconductor device has encountered a problem that, in transmission lines, high frequency components of data signals are attenuated to distort signal waveforms which causes data transmission errors. Accordingly, demands for using a pre-emphasis technology increases to compensate more properly for attenuation of high frequency components in accordance with characteristics of the semiconductor integrated circuit devices and transmission lines.
The term “pre-emphasis” means that the amplitude of a high frequency component of a data signal is preliminarily intensified in the transmitting side to compensate for attenuation of the high frequency component caused by the transmission lines. When pre-emphasis is performed, the transmitting side increases the amplitude of a high frequency component of a data signal and decreases the amplitude of a low frequency component of the data signal before transmitting the signal to the receiving side. The increasing the amplitude of a high frequency component is called “emphasis”, whereas the decreasing the amplitude of a low frequency component is called “de-emphasis”.
The expressions “emphasis signal” “emphasis voltage” “emphasis amplitude” and “emphasis processing” used hereinafter mean a signal which has been subjected to emphasis, a voltage of the signal which has been subjected to emphasis, an amplitude of the signal which has been subjected to emphasis, and an emphasis processing, respectively.
Similarly, the expressions “de-emphasis signal” “de-emphasis voltage” “de-emphasis amplitude” and “de-emphasis processing” mean a signal which has been subjected to de-emphasis, a voltage of the signal which has been subjected to de-emphasis, an amplitude of the signal which has been subjected to de-emphasis, and a de-emphasis processing, respectively.
The reason for applying de-emphasis to a low frequency component when performing pre-emphasis is that electric power required for emphasis can be reduced. This is because the purpose of pre-emphasis adjustment in the transmitting side made according to attenuation caused by transmission lines is to make the amplitude of a low frequency component agree with the amplitude of a high frequency component in the receiving side. Thus, as the amplitude of a low frequency component when the amplitude of the low frequency component agrees with the amplitude of a high frequency component becomes smaller, electric power required for emphasis can be reduced.
However, the emphasis processing and de-emphasis processing performed in pre-emphasis cannot be separately controlled from an economical viewpoint, so the de-emphasis amplitude is determined in accordance with the emphasis amplitude. Thus, no method can be used which preliminarily sets the amplitude of a low frequency component to a small value and then makes the amplitude of a high frequency component agree with that amplitude.
Consequently, to adjust pre-emphasis intensity properly, the value of pre-emphasis must be stepwise adjusted until the amplitude of a high frequency component agrees with the amplitude of a low frequency component in the receiving side.
FIGS. 12 to 14 are each a view for explaining the operation of pre-emphasis.
For example, when pre-emphasis intensity is insufficient with respect to the amount of attenuation of a high frequency component in the transmission line, the amplitude of a high frequency component of data signal is, as illustrated in FIG. 12, smaller than the amplitude of a low frequency component. As a result, the eye pattern opening of the data signal at the receiving end becomes small.
Meanwhile, when pre-emphasis intensity is excessive, the amplitude of a high frequency component of a data signal is, as illustrated in FIG. 13, larger than the amplitude of a low frequency component. As a result, the eye pattern opening of the data signal at the receiving end becomes excessively large.
Accordingly, pre-emphasis intensity must be adjusted to a proper value in accordance with the amount of attenuation caused by the transmission line. More specifically, as illustrated in FIG. 14, pre-emphasis intensity must be adjusted to a proper value so that the amplitude of a low frequency component of a data signal agrees with the amplitude of a high frequency component at the receiving end.
An example of this method of adjusting pre-emphasis intensity has been described in Japanese Patent application Laid-Open No. 2004-336407.
Referring to the above patent publication, at the data signal receiving end, the signal level of a high frequency component when the emphasis processing is turned on, is compared with the signal level of a low frequency component when the emphasis processing is turned off. Then, pre-emphasis intensity is adjusted at an output buffer circuit arranged in the data signal transmitting side so that the level difference therebetween is reduced to zero. In this case, when the emphasis processing is turned off, this means that the de-emphasis processing of the amplitude of a low frequency component is not performed, either. Consequently, according to the above patent publication, when the adjustment of pre-emphasis intensity is completed, at the receiving end, the amplitude of a high frequency component when the emphasis processing is turned on, agrees with the amplitude of a low frequency component when the de-emphasis processing is not performed.
However, when pre-emphasis is performed, as described above, the transmitting side applies emphasis to the amplitude of a high frequency component of a data signal and also applies de-emphasis to the amplitude of a low frequency component of the data signal before transmitting the signal to the receiving side. Consequently, according to the invention described in the above patent publication, when pre-emphasis is performed at the intensity used when the adjustment is completed, the amplitude of a high frequency component which has been subjected to emphasis may be excessively larger at the receiving side than the amplitude of a low frequency component which has been subjected to de-emphasis. When the difference between the amplitude of a high frequency component and the amplitude of the low frequency component becomes excessively large, waveform distortion occurs, thus leading to erroneous data determination in the receiving side.